I would check on the ASUS fora, however I have a harmless option for you to try, and because you have nothing to lose.... set your FSB to 334, instead of 333.
Our boards are similar in that for the sake of stability, it will take miniscule liberties with our settings. But for all its good intentions, there are some hardware confiigurations that result in an unbootable machine with FSB set to 333 or 377. Your rig is a candidate for this problem. As is mine, and I am running along happily at a setting of 334 for years now. In fact, if we set FSB in the range of 266-332, we can overclock as normal. Also 334-376 and 378-400+ work fine. But a FSB of 333 or 377, combined with rigid pci and (wait for it) 64 bit Vista (bizarre) simply will not work in some instances. Its wierd and annoying, but rare. I only mention it because you an use logical troubleshooting methods for eternity and not get it to work. And since there is nothing to lose, I suggest skipping 333 and giving 334 a shot.
Having said that, first pay attention to crispy2010's post just above this one. Especially about the RAM... first of all, it is common for 4x1G to give problems at the start, especially if you are trying to install a 64 bit version of Vista... but once you get it going, you can add the ram back and be okay. Also, if you have 4 sticks, you may need to up the northbridge (MCH) voltage just a smidge. (like 2 or 3 notches)... You're going to need to run memtest and prime blended in order to ensure your VDimm and MCH Voltages are giving your RAM enough juice. 4 cores, 4 sticks of ram, it can tax the Northbridge. In fact, I would suggest you raise the northbridge voltage even if you have only two sticks (1 or 2 notches), then back down until unstable.
Basically, do everything crispy said. Try skippinng 333 and set FSB to 334. and if that still doesn't work, look to
1. your CPU voltage... knock it up to 1.375 or 1.385 (you can back down later), and this will give you some headroom to focus on the likely culprit:
2. your RAM... Again, take Crispy's advice for safety, including ensuring your RAM divider is set properly (personally I think that you can use 1:1 and run your ram at 333 [edit: I meant 334]. Cpuz is good for checking that you got the timing right. (Its worth a check. I am far from stupid, and even have a masters in Physics... yet, it is as if BIOS makers rounded up the most intelligent people alive on this planet and hosted a conference dedicated to finding the most confounding method of relating the RAM speed to the FSB. This ratio business is crazy, even worse, some BIOS simply give clock speeds that may or may not be the speed the RAM will run at, depending on how you set your ratio! Is madness, and worth double checking).... but moving on, with Crispy's advise taken, lets stick with the RAM, which in this case includes:
3. raising the northbridge voltage three notches. If you can boot to Vista, back up one notch. Then test for at least 5 horus using Prime95 Blended. If that does not work, then
4. Forego the Auto setting for Vdimm and for your RAM, set the voltage for 1.9 manually, all else can remain at auto... or SPD, if you really want to be safe, loosen the first three timings from 4-4-4 to 5-5-5, but not at first. At first, leave the timings as is and see if we can boot.
5. and if THAT doesn't work, set Vdimm to 1.8, the recommended voltage for your RAM is 1.8-1.9 (unless you have some exotic Gskill DDR-800 RAM I haven't heard of. This is odd, as your spec voltage is a RANGE. For example, many ABIT owners cannot boot a new overclock unless the ram is precisely as spec'd. That your RAM is spec'd to use a RANGE from 1.8-1.9, instead of a single value, we need to try both) That was never a problem for me, but it happens. Personally, with a Quad, I always keep the ram at spec +.1 so mine works just fine at 1.9.
So... this should keep you busy for a while. If you get everything working, I suggest you leave Vdimm at 1.9, even though 1.8 may have been needed to get you going, 1.9 is just enough over 1.8 to ensure you have enough juice as PCWizard and Everest detect my Vdimm at about .5v less than my BIOS reports it. If you have 4 sticks its a no-brainer... set VDimm to 1.9.
Bear with me, but I just want to end with a statement simplifying all of this:
Despite the length of this post, for the Q6600 and your board, so long as your don't OC to 3.4Ghz or more, you really only need to worry about three voltages, in this order:
1. Vcore (we upped it a few notches to 1.375 or 1.385). Once stable, we can try lowering this back to the default (or auto), one notch at a time.
2. MCH (or whatever your Northbridge voltage is called), we took it up a few notches. If you have 4 sticks or ram, this is a no-brainer, with no need to raise the voltage 1 step at a time. Without ever exceeding recommended limits, simply raise MCH 3 notches. If you like, once stable, you can reduce this a notch, but I do not think you will ever have a stable Q6600 with 4GB of ram at stock MCH. Certainly if they are 4x1GBb.
3. Vdimm, which we set to 1.9, unless we needed 1.8 to boot the first time. If stable, don't lower this. leave it at 1.9 if you can.
Simple.
Crispy's advice, including the RAM divider, along with these 3 voltages, should easily boot with FSB set to 334. Especially since your temperatures are less than 75-80c. So you may want to check your temps while in the BIOS because if you can exclude overheating, including the hot as hell Radeon 4850, it should work.
Final note, the Radeon 4850 runs hotter than anything Ive ever seen. I put an aftermarket cooler on it and reduced the temps by HALF!!!! 75C IDLE and 90C LOAD are now just 36C IDLE and 48C under load!!! in other words, the 4850, as awesome as it is, needs to be watched for temperature just as much as your cpu in this instance.
Good luck!